1. Field of the Invention
The invention relates to an operational amplifier (OPAMP), and more particularly, to an operational amplifier of a multi-mode OPAMP-based circuit.
2. Description of the Related Art
Operational amplifiers (OPAMPs) have been applied extensively in the field of electrical devices and electronics, such as inverting amplifiers, integrators, and filter circuits, to name just a few instances. With the rapid scaling in CMOS processes, supply voltages in VLSL have been dramatically reduced in recent years. Acting as a fundamental block in most analog systems, operational amplifiers are required to achieve high gain and large bandwidth simultaneously in low-voltage applications. Since conventional cascode amplifiers, which increase the gain by stacking up transistors, are not suitable in low-voltage designs due to small voltage swings, more circuit designers are aware of the importance of multi-stage amplifiers, which boost the gain by increasing the number of gain stages horizontally. However, all multi-stage amplifiers suffer close-loop stability problems due to their multiple-pole nature in the small-signal transfer functions. Therefore, many frequency compensation topologies have been proposed to ensure the stability of the multi-stage amplifiers. Generally, the operational amplifier applied in the conventional driver chip is normally a two-stage amplifier having a first-stage amplifying circuit for gain enhancement and a second-stage output circuit for driving the capacitive or resistive load. However, multi-stage operational amplifiers are also gaining in popularity.
The most relevant characteristics of an amplifier circuit are usually gain and bandwidth. There is an inverse relationship between the gain and the bandwidth of amplifiers. In general, higher gain values are associated with lower bandwidths, and lower gain values are associated with higher bandwidths. Various compensation techniques, such as Miller compensation or Ahuja compensation, are known for adjusting the frequency of the poles of the amplifier. Miller compensation employs a feedback capacitor connected across an input and output of the second amplifier stage. In Ahuja compensation, a current gain device is added in the feedback loop of the second amplifier stage.